Radon Diffusion Measurement in Polyethylene based on Alpha Detection
Wolfgang Rau
Citation: AIP Conf. Proc. 1338, 164 (2011); doi: 10.1063/1.3579576
View online: http://dx.doi.org/10.1063/1.3579576
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Radon Diffusion Measurement in Polyethylene based on
Alpha Detection
Wolfgang Rau
Department of Physics, Queen’s University Kingston, ON, Canada K7L 3N6
Abstract. We present a method to measure the diffusion of Radon in solid materials based on the alpha decay of the
radon daughter products. In contrast to usual diffusion measurements which detect the radon that penetrates a thin barrier,
we let the radon diffuse into the material and then measure the alpha decays of the radon daughter products in the
material. We applied this method to regular and ultra high molecular weight poly ethylene and find diffusion lengths of
order of mm as expected. However, the preliminary analysis shows significant differences between two different
approaches we have chosen. These differences may be explained by the different experimental conditions.
Keywords: Radon, Diffusion in Polyethylene, Dark Matter
PACS: 66.30.je, 23.60.+e, 25.55.-e, 95.35.+d
INTRODUCTION
Modern experiments in Particle Astrophysics such as direct searches for Dark Matter in the form of Weakly
Interacting Massive Particles (WIMPs) which may constitute more than 80 % of the matter in the Universe, searches
for neutrinoless double beta decay, which may reveal the absolute mass scale and particle nature of the neutrino, or
measurements of low energy neutrinos from different sources are all searching for exotic events with low energies
occurring at a very low rate. As such they have very strict requirements regarding the radioactive background
radiation which may mimic the rare events searched for.
Radon (in particular 222Rn) poses a serious threat to such experiments due to its high mobility and the long half
live of its daughter products. Radon decays through a series of alpha and beta decays into the long lived 210Pb
(t1/2 = 22.3 years), which in turn decays via 210Bi to the alpha emitter 210Po (t1/2 = 138 days) until it reaches
eventually the stable nuclide 206Pb. Besides the direct radiation from the long lived daughter products, there is a
chance that the alpha decay of 210Po produces neutrons via (α,n) reactions in the material where it is deposited.
Neutrons are of special concern for Dark Matter experiments since the nuclear recoil produced by a neutron is
indistinguishable from a nuclear recoil induced by a WIMP.
In this paper we are specifically concerned about the interplay of radon with polyethylene, a material commonly
used in dark matter experiments as shield against neutrons. As a first step in determining the potential danger, we
investigate the amount of radon that accumulates when polyethylene is exposed to room air. In a later study we will
then determine what the neutron production rate from the accumulated radon would be, so we can pose a limit on the
allowed exposure of the shielding material to radon.
METHOD
We study the diffusion properties of polyethylene by first exposing the material to a high concentration of radon
for an extended period of time. Then we measure the alpha decays of the radon daughter products. Two different
incarnations of this method have been tested, one based on the long lived 210Po, and one relying on the short lived
products 218Po and 214Po; in this latter method also the radon itself is observed. This is different than the usually
Topical Workshop on Low Radioactivity Techniques - LRT 2010
AIP Conf. Proc. 1338, 164-167 (2011); doi: 10.1063/1.3579576
© 2011 American Institute of Physics 978-0-7354-0892-0/$30.00
164
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applied in-situ method of measuring the amount of radon that penetrates a thin barrier [1]. The advantage of our
method is that even with a moderately strong radon source it is possible to do the measurement by just increasing the
exposure time, while an in-situ measurement requires a fairly strong source. Further, with our first method we
directly map out the exponential activity distribution which reduces the sensitivity to systematic uncertainties. On
the other hand, the use of 210Po automatically involves long time scales (typically several months before the result
can be retrieved).
Long Lived Daughter: 210Po
For this method we prepare a sample of poly ethylene consisting of thin sheets which are carefully cleaned and
pressed together into a block of material. One side of this block is then exposed to radon which consequently will
diffuse through the different layers. After a certain time (typically a couple of weeks) the sheets are separated and
the remaining radon will quickly diffuse out. After an appropriate time during which the 210Pb that was built up
while the sample was exposed to radon produces 210Po, the individual poly ethylene sheets are exposed to an alpha
detector (ORTEC Dual Alpha Spectrometer 576A) and the 210Po concentration is determined.
Due to its long half life, the 210Po will not be in equilibrium with its parent 210Pb and the relative activity will
change over time. Since the individual measurements take of order of days, this change in relative activity has to be
taken into account properly. Also, we need to consider that 210Po is started to be produced already during the radon
exposure. This is done by a diffusion and decay model which calculates the 210Po activity for any point in space and
time for a given radon exposure and given diffusion parameters (we consider the diffusion constant and the
solubility, which determines the ratio of the radon concentration on either side of the air-poly ethylene interface).
The diffusion parameters are extracted by varying their values in the model until the best fit to the measurements is
found.
In addition we determine the efficiency of the alpha detection with a Monte Carlo simulation, based on the
simulation package SRIM [2].
Short Lived Daughters
For this method we start the measurement immediately after the end of the exposure of the sample to the high
radon concentration. The alpha decays of the short lived daughter products are observed as the radon diffuses out of
the material. In this case we only measure one sheet for each sample. We take advantage of the different live times
and alpha energies of the involved nuclides by splitting the data into four energy bins. The spectrum changes with
the decay of the different species as well as with the change in the depths profile due to the diffusion. We again
compare the measurement to a model calculation, but in this case we only extract the diffusion constant. Additional
free parameters in the fit are a potential enhancement of radon daughter products on the material surface and the
time between end of exposure and the start of measurement (this is particularly critical for the surface contamination
with 218Po).
RESULTS
We applied this method to two types of samples: regular poly ethylene (PE) and ultra high molecular weight poly
ethylene (HMWPE). In figure 1 you see a comparison between the measurement and the fit for the first method
applied to the HMWPE sample. For the purpose of this visual presentation only, the measured values are converted
to the equivalent 210Pb concentration at the end of the radon exposure (the fit is done based on the original
measurements). The fit quality is reasonable and the extracted parameters are of order of mm as expected for soft
polymers [1].
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FIGURE 1. Radon diffusion in HMWPE. This measurement is based on the alpha decay of 210Po. To compare the data points
with the model calculation (solid line) they have been converted to the equivalent activity of 210Pb at the end of exposure. The fit
is reasonable and the extracted diffusion parameters are in the expected range.
In figure 2 you see our model compared to the measurement of the short lived products in HMWPE. The four
curves represent the four energy bins of our analysis. In contrast to the first method where the activity is plotted as
function of the distance from the surface, here we consider the rate as function of time as the radon diffuses out of
the material.
FIGURE 2. Radon diffusion in HMWPE. This measurement is based on the short lived radon decay products 218Po and 214Po as
well as on 222Rn itself. To take advantage of the different decay energies and half lives the data have been split in four energy
bins as indicated in the legend, before comparing to the rates predicted by the model.
In the table below we summarize our results; the uncertainties are determined by mapping out the χ2-surface and
finding the values where the reduced χ2 is one unit above the value of the best fit. Note that the uncertainties in
solubility and diffusion constant are correlated and the resulting uncertainty in the total amount of radon
accumulated for a given exposure is smaller than a naïve combination of the uncertainties would imply.
Please note further that all results and uncertainties presented in this paper are preliminary and require further
refinement, however neither the central values nor the uncertainties are expected to change significantly.
166
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TABLE. Diffusion parameters for PE and HMWPE as measured with the two different methods described in
this paper. For a discussion of possible reasons for the differences between the results of the two methods see
text.
210
Short lived
Sample
Po
Diff.
length
Diff.
Const.
Diff. length
Diff.
Const.
Solubility
[mm]
[10-12 m2/s]
[mm]
[10-12 m2/s]
0.68 ± 0.10
1.2 ± 0.2
0.76 ± 0.06
6.0 ± 0.8
1.7 ± 0.1
PE
3.7+1.4/-1.0
0.45 ± 0.07
1.3 ± 0.20
7.1 ± 0.8
1.8 ± 0.1
HMWPE
We notice that the diffusion constants found by the two different methods do not agree. There are a number of
possible reasons for this observation: it is known that humidity can greatly influence the diffusion of radon in
polymers [1]. The humidity of the samples was not controlled in this measurement, which may lead to differences in
the results from the two methods. Another possible contribution to the differences may be the fact that the samples
are under vacuum while exposed to the alpha detector. For the first method (210Po measurement) the radon diffuses
into the sample under normal pressure, while for the second method (short lived decay products) the radon diffuses
out of the material under vacuum. The low pressure may pull other gases out of the sample which in turn could
enhance the diffusion of the radon atoms. With the available data no further conclusion about the origin of the
differences can be drawn.
CONCLUSION
In this paper we have presented two methods for the measurement of radon diffusion in polymers based on the
detection of alpha decays from radon daughter products. The diffusion parameters are extracted based on the
comparison of the measured data with a diffusion and decay model and a Monte Carlo simulation of the efficiency
of the detection of the alphas from the different daughter products. We find a reasonable agreement between the
behavior predicted by the model and the measurements; however, we find significant differences in the extracted
values between the two methods, which may be explained by the different experimental conditions.
ACKNOWLEDGMENTS
I would like to thank the undergraduate students and the visiting graduate student who helped me with these
measurements: Brian Drover, Mina Rohanizadegan, Graham Edge and Hai Jun Cho. I would further like to thank
Dr. Anthony Noble for allowing me to use his alpha detector. This project was supported by NSERC.
REFERENCES
1. M. Wójcik, W. Wlazło, G. Zuzel, G. Heusser, Nucl. Instrum. Methods A 449 (2000) 158
2. J. Ziegler, SRIM: see http://www.srim.org/ (2008)
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